A hot melt adhesive is a solvent-less adhesive, which is heat-melted, applied to an adherend, and then solidified by cooling to exhibit adhesion, and thus a hot melt adhesive enables instantaneous adhesion and rapid adhesion, and is applicable in wide variety fields such as paper processing, woodworking, hygienic materials, and electronics.
As a base polymer for the hot melt adhesive, for example, an ethylene-based copolymer such as an ethylene-vinyl acetate copolymer (hereinafter also referred to as “EVA”) and an ethylene-ethylacrylate copolymer (hereinafter also referred to as “EEA”), an olefin-based resin such as polyethylene, polypropylene, amorphous polyalphaolefin (hereinafter also referred to as “APAO”), a styrene-based block copolymer (e.g., styrene-isoprene-styrene-based block copolymer (hereinafter also referred to as “SIS”) and styrene-butadiene-styrene-based block copolymer (hereinafter also referred to as “SBS”)) and a synthetic rubber of hydrogenated product thereof, and polyurethane are widely used, depending on intended use of the hot melt adhesive.
Among these hot melt adhesives, a hot melt adhesive including an ethylene-based copolymer as base polymer is often used in a paper processing field such as book binding and packaging, and a woodworking field.
For application of a hot melt adhesive, a dedicated coating device called hot melt applicator is often used. The hot melt applicator has a nozzle, i.e., a discharge opening. The hot melt adhesive heated to about 120 to 190° C. is discharged from the tip of the nozzle for application to an adherend. In application of the hot melt adhesive, a hot melt adhesive string may sometimes occur between the tip of nozzle and the adherend. The occurrence of string is due to the stringing properties of a hot melt adhesive, soiling the nozzle and the adherend. Accordingly, adhesive manufacturers have important responsibility for developing a hot melt adhesive having less stringing.
In Japanese Patent Laid-Open No. 2008-527067 and Japanese Patent Laid-Open No. 2008-214539, ethylene (polyolefin)-based hot melt adhesives with the purpose of reducing stringing are described. Japanese Patent Laid-Open No. 2008-527067 discloses a hot melt adhesive including ethylene/C3-C20 α-olefin copolymer as base polymer, for use in packaging, book binding, woodworking and the like. Japanese Patent Laid-Open No. 2008-214539 discloses an olefin hot melt adhesive which includes a “stringing reducing agent” which is formed by modifying an olefin with unsaturated polycarboxylic acid, or acid anhydride or ester thereof.
Reducing stringing of a hot melt adhesive has been more severely required year by year. Particularly, users of thick paper such as corrugated cardboards and cartons earnestly require the reduction in stringing. Although the hot melt adhesives of Japanese Patent Laid-Open No. 2008-527067 and Japanese Patent Laid-Open No. 2008-214539 have reduced stringing properties, it cannot be said that the performance severely required by the users is sufficiently satisfied with the adhesives.
Japanese Patent Laid-Open No. 2012-177009 discloses an ethylene-based hot melt adhesive in which the composition of ethylene/carboxylic acid ester is adjusted to a specific range and which can thus reduce stringing at a high level and which has excellent heat resistance and thermal stability. However, demands for hot melt adhesives from users have been increasing over the years. The hot melt adhesive of Japanese Patent Laid-Open No. 2012-177009 cannot be said to satisfy the users' high demand for properties of adhesion to paper.
Also, cartons having a surface coated with a chemical have been frequently used to package products recently to add a high quality feeling to the packaging of products. Therefore, hot melt adhesives are required to have a certain level of adhesion even to the coating surface of cartons. It is an urgent need in the hot melt adhesive industry to develop an ethylene-based hot melt adhesive capable of reducing stringing, having excellent thermal stability and also having excellent properties of adhesion to paper.